(1) Field of the Invention
The invention relates to data transfer systems. More specifically, the invention relates to arbitration of access to a serial bus.
(2) Related Art
For high speed data transfers over a distance, various high speed serial data transfer mechanisms have been developed. The IEEE Standard for a High Performance Serial Bus, IEEE Std. 1394-1995 published Aug. 30, 1996 (1394-1995 Standard) and its progeny provide a high speed serial protocol which permits implementation of high speed data transfers. The existing progeny includes P1394a Draft Standard for a High Performance Serial Bus (1394a Standard) and P1394b Draft Standard for a High Performance Serial Bus (1394b Standard). Generically, systems implementing 1394-1995, 1394a, 1394b or subsequent revisions and modifications thereof are referred to herein as 1394 systems.
In 1394-1995 asynchronous arbitration is implemented using a simple timing strategy in which arbitration cannot begin until the bus has been idle for a predetermined period of time. This predetermined time period is known as a subaction gap. The subaction gap may vary from less than 0.5 microsecond to more than 10.5 microseconds depending on the size of the bus. The subaction gap is set to be greater than the worst case round trip propagation delay time across the bus. Accordingly, requiring the wait for a subaction gap before arbitration begins ensures that no node connected to the bus will begin arbitrating after a packet is transmitted before an acknowledgment packet (ACK) has had the opportunity to be transmitted and received. Unfortunately, the 1394-1995 Standard makes no distinction between an ACK and the other packets. Thus, after an ACK the subaction gap represents purely wasted bandwidth. Accordingly, one arbitration acceleration mechanism is ACK-accelerated arbitration. In ACK-accelerated arbitration when an ACK is seen on the bus arbitration can begin immediately without waiting for a subaction gap.
The 1394-1995 provides for one efficiency enhancement mechanism, basic concatenation. In basic concatenation when the first node sends a request to a second node, and the second node sends the first node an ACK, if the second node can access the data requested by the first node quickly enough it holds the bus after the ACK and concatenates the response packet onto the ACK. Similarly, 1394-1995 provides for concatenation of isochronous subactions sent by the same node. Fly by arbitration is a matter of expanding the scope of the basic concatenation mechanism. Particularly, when a node has a packet ready to transmit and is waiting to arbitrate for the bus and receives an unrelated ACK, the node merely concatenates its unrelated packet to the end of the ACK. The node then transmits the ACK and the concatenated packet out the non-receiving ports and only the concatenated packet out of the receiving port. An analogous fly by procedure applies to isochronous transactions.
If the ACK is received on an parent port it is likely that it is also being simultaneously received by another node. Therefore, in such case fly by arbitration is likely to result in packet collision which should be avoided. However, receiving a packet on a child port permits fly by arbitration either in the case where the received packet is an ACK or the received packet is an isochronous packet. Since the 1394 protocols make no guarantee about the order of packet transmission, as long as all nodes get to transmit asynchronous packets during a fairness interval and isochronous packets within an isochronous cycle, the bus is well behaved. ACK-accelerated and fly by arbitration improve bus efficiency in the case of ACKs and isochronous packets. However, a significant number of subaction gaps are still likely as several other asynchronous ackless subaction types exist. Among existing ackless subactions are: broadcast, self-ID, link on, asynchronous stream, and PHY ping subactions. The ackless subactions are particularly prevalent when the internet protocol is mapped onto the 1394 protocol. Additionally, new packet types can be expected to be developed from time to time. Taken together these factors will cause enforced subaction gaps to lower overall bus efficiency.
In view of the foregoing it would be desirable to develop improved arbitration enhancements to further reduce the inefficiency caused by subaction gaps.
A method and apparatus for improving bus efficiency is disclosed. A determination is made if a packet to be transmitted is a last packet of a subaction. An end of subaction token is inserted at an end of the packet if the packet is a last packet of a subaction. The packet is then transmitted out at least one port.